In a typical conventional electronic blood pressure meter (for instance, of an oscillation type), after a cuff is wrapped around an arm of a person and is initially pressurized to a certain level, the cuff is gradually depressurized. During this depressurization process, a DC component of the cuff pressure signal is detected and a pulse wave component contained in the cuff pressure signal is extracted at the same time. A maximum amplitude of this pulse wave signal is obtained, for instance for each pulse and an envelope of the maximum amplitudes arranged in time series (cuff pressure series) is obtained. Two levels of blood pressure are determined from this envelope and the DC component of the cuff pressure according to a certain algorithm. A typical algorithm for determining blood pressure values consists of determining an average blood pressure from the cuff pressure corresponding to the maximum value of the parameter (envelope), a systolic blood pressure from the cuff pressure of a high pressure side when the parameter corresponds to 50% of the maximum value, and a diastolic blood pressure from the cuff pressure of a low pressure side when the parameter corresponds to 70% of the maximum value.
In such an oscillation type electronic blood pressure meter, the pressure sensor can obtain an accurate distribution of the parameter only when it is attached to a proper position in an upper arm of the person and kept in the same position throughout the measurement. Therefore, when artifacts such as the motions of the arm or the body should arise during measurement, the distribution of the parameter as detected becomes distorted (See FIG. 5) and accurate determination of blood pressures according to the normal algorithm becomes impossible. This is highly inconvenient because a great care is required for blood pressure measurement and, yet, inaccurate measurements could still occur without being noticed.
Japanese Patent Laid Open Publication No. 61-196936 filed by the assignee of the present application discloses an electronic blood pressure meter which detects artifacts during measurement and permits accurate measurement of blood pressures. This electronic blood pressure meter comprises first determination means for comparing a current parameter value obtained by a pulse wave parameter extraction means with a previous parameter value and determining whether the current parameter value is greater than the previous parameter value or not, first difference computing means for computing a first difference value between the current parameter value and the previous parameter value, second determination means for determining whether the first difference value is greater than a predetermined value or not, second difference computing means for computing a second difference value between the previous parameter value and the yet previous parameter value, and third determination means for determining whether the second difference value is substantially greater than the first difference value or not, and the parameter value is determined to be abnormal when all the outputs from the first, the second and the third determination means are affirmative.
According to this previously proposed electronic blood pressure meter, the current parameter value is estimated to be abnormal when the first difference value obtained by the first computing means is determined to be greater than the predetermined value by the second determination means. Further, if the second difference value computed by the second computing means is determined to be substantially greater than the first difference value by the third determination means, it means that the parameter value has just sharply increased and it is estimated that an artifact may have affected the current parameter value. It also means that the measurement was not accurate and is required to be started all over again.
However, this method is based on the comparison of a plurality of pulse wave amplitudes or pulse wave parameter values and, therefore, a sufficient number of amplitude values must be made available before abnormal conditions (artifacts) can be detected. Therefore, a considerable time must be spent before being able to determine whether the measurement has been carried out in proper manner or not. Furthermore, the algorithm for these comparison processes is so complex that the response time of the machine or the microprocessor tends to be long and the cost of the electronic blood pressure meter increases due to added complexity of the hardware. Furthermore, criteria of the comparison processes may not be appropriate for all the persons whose blood pressures are to be measured and it was indeed found by the inventor that accurate detection of artifacts is sometimes not possible for certain individuals.